1. Field of the Invention
This invention relates in general to an uninterruptible power supplies, and more particularly to an AC transfer switch for switching between a first line cord and a second line cord.
2. Description of Related Art
Modem-day communication and computer equipment utilize solid state integrated circuits technology to transmit and control data and switching information. Circuits utilizing solid state integrated circuit technology are highly susceptible to variations in electric power from some desired standard. The power to energize these circuits can be readily derived from commercial AC power. The signal consistency of commercial AC power, however, is unreliable due to user demand and other extraneous considerations causing power level fluctuations. These signal variations can take the form of blackouts, brownouts, or transient interruptions or surges. Power outages that extend for a long period of time, on the order of seconds and longer, can render computer peripheral equipment as unavailable.
Typical subsystems are designed to ride through power outages that last up to 20 milliseconds. If the subsystem is designed for battery backup, the system can continue to operate for several minutes before the unit shuts off due to the batteries discharging. Unfortunately, some power outages can last for an extended period of time beyond the holdup capabilities of the batteries. Therefore, customers have required subsystems that are designed to accommodate two separate incoming power line cords. With two separate incoming power line cords, in the event of a power loss on one power cord, the second power cord will deliver the necessary power to keep the subsystem operational. The main reason for this requirement is that customers want the most reliable equipment that they can get due to the high costs, to the customer, associated with the subsystem being down.
Previous designs have also provided increased fault tolerance. To overcome the problems with the single power line cord, subsystems have been designed with a 2N power system. In other words, each power line cord would have its own separate power system, i.e., twice the reliability. Therefore, in the event of a power outage on one line cord, the second line cord's power system is capable of powering the entire subsystem.
Unfortunately, a 2N power system adds additional cost to the product. For example, a fully configured subsystem may require ten AC/DC power supplies to meet the specified power requirements. However, each of the AC/DC power supplies may cost $1,000. To accommodate a 2N power system, an additional 10 AC/DC power supplies would be needed thereby increasing the overall cost by $10,000, excluding packaging, cabling and other miscellaneous costs.
If an AC switch was used to transfer the power from one line cord, during an extended power outage, to a second line cord, only two extra AC/DC power supplies would be needed to accommodate an N+1 power system. A N+1 power system includes a redundant power supply for additional system reliability. Since the AC switch typically cost approximately $300, the overall savings would easily approach $10,000 per fully configured subsystem.
Nevertheless, the major problem with designing an AC switch, for application in computer peripheral equipment, is twofold. First, the AC switch has to be fault tolerant to meet the product safety requirements. When power is being supplied to a subsystem by one line cord, the second line cord can not be connected to the first line cord's power source. Second, the AC switch needs to operate in such a manner that the subsystem will continue to operate non-destructively. In other words, the subsystem must continue to operate normally, without shutting down, when the switch changes from one line cord to the second line cord.
It can be seen then that there is a need for an AC transfer switch that is designed to be fault tolerant.
It can also be seen that there is a need for an AC transfer switch that will ensure that continuous operating power is supplied to a load during the transfer process.